Conventional measuring equipment is known which is configured as shown in FIG. 20. That is, in this equipment, a signal S to be measured is inputted to a sensor 100, and an amplifier 101 then amplifies an output signal from the sensor 100 to a predetermined level. Furthermore, a sensor sensitivity adjusting circuit 102 amplifies an output signal from the amplifier 101 to a desired level compatible with the sensitivity of the sensor 100. Then, a processing section 103 executes processes such as an effective value operation and A/D conversion on an output signal from the sensor sensitivity adjusting circuit 102. Finally, a display 104 displays an output signal from the processing section 103 as a measured value L.
Further, a signal processing circuit used in the conventional measuring equipment is configured as shown in FIG. 21. That is, in this circuit, in order to deal with inputted analog signals with a wide range of amplitudes, an amplifying circuit 200 and an attenuating circuit 201 are connected together in parallel to simultaneously process the analog signals. When the analog signal has a small amplitude, a signal amplified by the amplifying circuit 200 is mainly employed. On the other hand, when the analog signal has a large amplitude, a signal attenuated by the attenuating circuit 201 is employed. These signals are subjected to A/D conversion by A/D converters 202 and 203, and then the converted signals are synthesized with each other by a processor 204 provided with a synthesizing circuit. Reference numeral 205 denotes an amplification factor adjuster, and reference numeral 206 denotes a microphone. Reference numeral 207 denotes an input amplifier, and reference numeral 208 denotes a display.
Further, a calibration operation for the signal processing circuit of the measuring equipment is essential in determining criteria for output signals from a sensor or the like. A calibration circuit generally has a sensor sensitivity adjusting circuit that absolutely calibrates the sensitivity of the sensor and an internal calibration circuit that subjects the results of the absolute calibration to self-diagnosis to check whether or not the measuring equipment has been calibrated.
Furthermore, the measuring equipment provided with the calibration circuit comprises, for example as shown in FIG. 22, a microphone 300, an input switch 301, an amplifier 302, a sensor sensitivity adjusting circuit 303 composed of a variable resistor, an A/D converter 304, a processing section 305 having an effective-value calculating circuit, a display 306, an internal-calibration signal oscillator 307, an internal-calibration signal adjusting circuit 308 composed of a variable resistor, and other components.
First, absolute calibration is carried out as described below. An analog signal from the microphone 300 is inputted. The inputted analog signal is inputted to the sensor sensitivity adjusting circuit 303 via the input switch 301 and the amplifier 302. Then, the sensor sensitivity adjusting circuit 303 adjusts the amplitude of the analog signal to a predetermined value and then inputs the adjusted signal to the processing section 305 via the A/D converter 304. Then, the display 306 displays the result of calculation executed by the effective-value calculating circuit possessed by the processing section 305. At this time, when a predetermined value is displayed as the result, the absolute calibration is completed.
Then, internal calibration is carried out as described below. The internal-calibration signal oscillator 307 outputs an internal-calibration signal. This internal-calibration signal is inputted to the processing section 305 via the internal-calibration signal adjusting circuit 308, the input switch 301, the amplifier 302, the sensor sensitivity adjusting circuit 303, and the A/D converter 304. Then, the effective-value calculating circuit of the processing section 305 calculates an effective value. This effective value is displayed on the display 306. The internal-calibration signal adjusting circuit 308 then adjusts this displayed value to a predetermined value, thus completing the internal calibration.
This calibration method is easily carried out by adjusting variable resistance.
Furthermore, in a conventional acoustic measuring instrument, an input signal loaded through an A/D converter is processed by a DSP (Digital System Processor), and the processed signal is outputted through a D/A converter. However, since a voltage proportional to the input signal is outputted from an output terminal while absorbing differences in the characteristics of components such as resistors and D/A converters, an electric circuit 400 such as the one shown in FIG. 23 is generally inserted between a D/A converter 401 and an output terminal 402.
Inserting the electric circuit 400-allows an input signal x and an output terminal voltage y to have a proportional relationship as shown in FIG. 24. In this case, y=ax+b represents an I/O relationship in an actual circuit, and y=a0x+b0 represents a desired I/O relationship.
If the desired I/O relationship is established by adjusting a variable resistance VR1 in the electric circuit 400 so as to make the inclination a of the straight line closer to the desired value a0 and adjusting a variable resistance VR2 so as to make the intercept b of the straight line y closer to the desired value b0.
However, a change in variable resistance VR1 actually changes the intercept b.
Accordingly, during an actual operation, to adjust the inclination a so as to make it closer to the desired value a0, it is necessary to input two types of reference signals R1 and R2 the magnitudes of which are different and then to determine the inclination from the difference between outputs (M1-M2).
Thus, in the prior art, the inclination a of the straight line is adjusted to the desired value a0 while repeatedly and alternately inputting the two types of reference signals R1 and R2 and fine-tuning the two variable resistances VR1 and VR2.
Further, an acoustic measuring instrument is normally provided with a level range switch used to select an optimum maximum input level according to the level of an input signal to be measured.
However, an error is sure to occur in amplification factor for each range of the level range switch owing to differences among resistors constituting amplifiers provided for the respective ranges. Such errors can be adjusted to within a desired error range by managing the resistance values of the resistors constituting the amplifiers.
Thus, adjusting resistors may be provided in order to reduce errors in amplification factor caused by the level range switch.
Similarly, the acoustic measuring instrument is provided with a frequency characteristic switch used to select a correction circuit that corrects the frequency characteristic of an input signal to be measured.
However, an error is sure to occur in amplification factor for each range of the frequency characteristic switch owing to differences among resistors constituting correction circuits provided for the respective ranges. Such errors can be adjusted to within a desired error range by managing the resistance values of the resistors constituting the amplifiers.
Thus, adjusting resistors may be provided in order to reduce errors in amplification factor caused by the frequency characteristic switch.
However, in the conventional measuring equipment shown in FIG. 20, no consideration is given to the effects on measured values of self-noise N from the sensor 100, amplifier 101, sensor sensitivity adjusting circuit 102, or the like. Accordingly, if an input signal S to the sensor 100 which is to be measured is small in magnitude, the self-noise N produces marked effects. This makes measurements impossible, resulting in deviation from the measurement range.
That is, as shown in FIG. 3, with the conventional measuring equipment, the relationship between the input signal S to the sensor 100 and a measured value L displayed on the display 104 remains linear if the input signal S exceeds a certain level Se. However, if the input signal S has the level Se or lower, the self-noise N contributes to eliminating the linearity. Consequently, input signals S at the level Se or lower are beyond the measurement range and cannot be measured.
In the figure, if the input signal S has the level Se or lower, the solid line indicates an actual measured value, whereas the dotted line indicates an ideal measured value.
Further, in the signal processing circuit shown in FIG. 21, input analog signals are amplified by the amplifying circuit 200 and attenuated by the attenuating circuit 201. These signals are then subjected to A/D conversion to obtain digital signals. The synthesizing circuit then converts and synthesizes the signals according to the amplification factor of the amplifying circuit 200 and the attenuation factor of the attenuating circuit 201. It is thus necessary to adjust the ratio of the amplification factor of the amplification circuit 200 to the attenuation factor of the attenuating circuit 201 to a predetermined value.
Thus, during an actual adjusting operation, while checking the displayed value shown on the display 208, the variable resistances constituting the amplification factor adjuster 205 must be adjusted to make the displayed value closer to a specified value. Consequently, the operation is not efficient.
Further, with the measuring equipment shown in FIG. 22, during an actual calibration operation, while checking the displayed value shown on the display 306 of the measuring equipment, the variable resistances must be adjusted to make the displayed value closer to a specified value. Consequently, the operation is not efficient.
Furthermore, if variable resistors are used, which are important elements that determine the performance of the measuring equipment, then the adjusted value may be varied by an external factor such as vibration. Thus, the use of variable resistors may result in instability.
Moreover, if the electric circuit 400 such as the one shown in FIG. 23 is inserted between the D/A converter 401 and the output terminal 402, an operator must regulate each acoustic measuring instrument and fine-tune the two variable resistances VR1 and VR2 many times. As a result, a large number of adjustment steps are required.
Further, if the acoustic measuring instrument is provided with a level range switch used to select the optimum maximum input level according to the level of an input signal to be measured, then errors in amplification factor caused by the level range switch and a frequency characteristic switch are not fully adjusted. It has thus been desirable to substantially zero switching errors associated with such range switching.
The present invention is provided in view of these problems of the conventional technique. It is thus an object of the present invention to provide a method of automatically correcting self-noise which method can lower the lower limit value of a measurement range by reducing the effects of self-noise on measured values, as well as an apparatus using this method.